a. Field of Invention
This invention pertains to an implantable cardioversion device having circuitry for shaping and controlling precisely the therapeutic pulses delivered to a patient. Alternatively, the device may be used to induce tachycardia or fibrillation.
b. Description of the Prior Art
In the present invention the term implantable cardioversion device or ICD is used to generically cover all implantable devices capable of delivering therapeutic pulses or shocks. That is, the term is intended to cover not only devices arranged and constructed to deliver cardioversion pulses, defibrillation shocks or other similar antitachycardia therapy but pacing pulses on demand for bradycardia therapy.
ICDs presently available on the marked deliver either pulses of defined duration and magnitude having relatively small energy content (in the order of 50 Microjoules) or high voltage energy content of up to 40 Joules. In either case, the energy for these pulses or shocks is derived from one or more capacitors. These capacitors are first charged to a nominal voltage, and when pulses are required, they are switched to the delivering electrodes and are discharged through the body tissues. While the defibrillation pulses are applied the capacitors cannot be charged back to their nominal value.
A further disadvantage of the present devices is that once the capacitors are switched to the electrodes, the currents generated are strictly depending on the impedance of the body and any parasitic resistivity of the electrodes. The magnitude of these currents is not otherwise limited or controlled.